(1) Field Of The Invention:
This invention relates to the electric cooking art and particularly to the use of etched foil heaters with a solid utensil-supporting cover plate.
(2) Description Of The Prior Art:
Solid plate surface heating units and cooktops have been proposed with high temperature glass-ceramic plates having film heaters of noble metal bonded directly to the underside of the plates. Three examples of glass-ceramic heating units using film heaters are shown in the two prior Hurko Pat. Nos. 3,067,315 and 3,883,719, both of which are assigned to the present assignee, and in the Brouneus Pat. No. 3,813,520. Such film heaters are of serpentine shape, and they are bonded directly to the plate. They provide a most efficient heating system for glass-ceramic surface heating units or cooktops because the film strips have a very low thermal mass and good thermal coupling with the plate, resulting in quick response to heat-up and cool-down conditions. The film heater stores very little heat, and it radiates very little heat in a downward direction because of its low emissivity surface. One deterrent to the use of the film heater designs for solid plate surface heating units is the relatively high cost of film materials because they are of noble metals, such as gold and platinum.
Etched foil heaters for use with solid plate surface heating means at low temperatures below about 450.degree. F. have been available from Safeway Products, Inc., of Middletown, Conn. Such foil heaters are highly reliable at relatively low temperatures, and their cost is a great deal below film heaters. An example of etched foil heaters is given in the Howie Pat. No. 3,869,596, which is assigned to Safeway Products, Inc. This patent shows a glass-ceramic plate with an etched foil heater bonded between two layers of dielectric material, and this laminated heating element is bonded directly to the underside of the glass-ceramic plate. As stated in this Howie patent, it has an anticipatd maximum operating temperature on the order of 450.degree. F.
This bonding action of the laminated foil heating element directly to the cover plate is a deterent because it limits the kind of foil material that may be used to a low thermal expansion metal foil so as to be able to match the coefficient of thermal expansion of the glass-ceramic plate, and this in turn limited the application of etched foil heaters to a relatively low temperature range having a maximum of about 450.degree. F. Moreover, prior art foil heaters used a flexible, thermosetting organic adhesive to make a strong bond, which again limited the foil heater to low temperature applications. At higher temperatures, such adhesive would carbonize and cause short circuits between the adjacent turns of the foil heater as was experienced during testing of the prior art devices.
The before mentioned U.S. Pat. No. 4,032,750 of the present inventor, describes an etched foil heater that is separate from the utensil-supporting cover plate and is for use at high temperatures in the vicinity of 1250.degree. F. That prior design had means to allow the resistive foil heater to freely expand at operating temperatures with relation to its supporting laminations of insulation. That prior design also utilized a high temerature inorganic binder for bonding the resistive foil heater to the dielectric substrate. The dielectric substrate was positioned directly against the underside of the cover plate but not bonded thereto, and the resistive foil means was adhesively bonded to the underside of the dielectric layer. The preferred embodiment of this prior heating unit had a second dielectric substrate bonded to the underside of the resistive foil means as well as to the first dielectric layer to form a flexible, laminated heating element.
The principal object of the present invention is to provide a solid plate surface heating unit with a separate, flexible, insulated foil heater for use at high temperatures in the vicinity of 1250.degree. F., where the foil is free to expand with a ceramic fiber paper substrate during thermal cycling of the heater.
A further object of the present invention is to provide a solid plate surface heating unit of the class described using an inorganic cement having high thermal conductivity and high electrical resistivity so as to provide increased heat flow toward the utensil-supporting cover plate.